1. Field
The present invention relates to a hybrid stepping motor with improved core shaped and winding structure.
2. Background
In recent years, various hybrid stepping motors with built-in permanent magnets have been researched and developed. Compared to general stepping motors, such hybrid stepping motors are advantageous in that they consume nearly half of electric power to generate the same torque as that of a general stepping motor.
FIG. 1 shows a hybrid stepping motor disclosed in Korean Patent Application Publication No. 1998-046512 as one example of known hybrid stepping motors. Referring to FIG. 1, in the prior art hybrid stepping motor 1, a rotating shaft 10 is rotatably supported on a pair of bearings 2, 3 disposed in both end portions of a stator casing 14. Ring-shaped first and second stator yokes 16, 16A with a ring-shaped magnet plate 30 mounted therebetween are disposed axially in the stator casing 14. Primary and secondary coil winding portions 20a, 21a are formed on an inner periphery of the first and second stator yokes 16, 16A, respectively. Primary and secondary coils 22, 23 are wound in the coil winding portions 20a, 21a, respectively. Thus, two cores are formed in the first stator yoke 16 while the other two cores are formed in the second stator yoke 16A. Surfaces of each core are provided with a plurality of stator teeth 17, 17a. The stator teeth 17, 17a, which are provided on the surfaces of the four cores of the first and second stator yokes 16, 16A, are deviated by a ¼ tooth for one another and with 90° phase difference. A rotor yoke 19 is disposed on the rotating shaft 10 to correspond to the stator yokes 16, 16A. An outer periphery of the rotor yoke 19 is provided with a plurality of rotor teeth 19a. 
In the above-described prior art hybrid stepping motor 1, rotation of the rotor yoke 19 continues while magnetic flux is sequentially focused on the four cores in a manner that one magnetic flux generated by a current alternately applied to the primary and secondary coils 22 and 23 and the other magnetic flux generated by the ring-shaped magnet 30 are overlapped or offset to each other.
However, in the above-discussed prior art hybrid stepping motor 1, only one of the four cores of the stator yokes 16, 16A contributes to torque generation. Specifically, when a current is applied to the primary coil 22, no current is applied to the secondary coil 23. In such a case, bias flux, which is caused by the ring-shaped magnet plate 30, is allowed to flow through air gaps existing at the second stator yoke 16A side, to which a current is not applied. Such bias flux flows particularly through the teeth, which is less deviated from the stator teeth 19a of the second rotor yoke 19, of two cores of the second stator yoke 16A. Accordingly, there is a problem with the prior art hybrid stepping motor 1 in that torque is produced in an opposite direction to a rotational direction of the rotor yoke 19, thereby deteriorating efficiency of the hybrid stepping motor 1. In addition, since the coil winding portions 20a, 21a and the coils 22, 23 of the stator yokes 16, 16A are located outwardly of the rotor yoke 19, there is a further problem in that increasing a diameter of the rotor yoke 19 is restricted due to a determined diameter of the hybrid stepping motor 1.